1. Field of the Invention
This invention relates to novel covering materials for controlling the growth of plants which are highly valuable in large-scale horticulture and private gardening.
2. Description of the Related Art
The primary purpose of the covering materials used in large-scale horticulture and private gardening is to decrease the thermal loss from the inside of the greenhouse atmosphere into the open air (i.e., reduce the rate of ventilation) and thereby achieve proper control of the temperature environment, regulation of the cultivation period, shortening of the time required for cultivation, increase of the number of times of cultivation, and the like. In addition, such covering materials have the effect of shutting out rain and thereby preventing the agricultural chemicals and fertilizers from migrating to the outside of the greenhouse, and also permit carbon dioxide application.
Since primary covering materials are exposed to the open air and sunlight, it is desirable that they have a long lifetime. Moreover, they must have high light transmittance. For this purpose, glass, plastic plates and films have been used. In particular, films have the advantage that they are cheap and can be attached or detached easily. On the other hand, a cheese cloth is frequently used inside the greenhouse for thermal insulation and shading in hot weather.
Thus, the primary function of covering materials currently used for practical purposes is to maintain and control the temperature environment.
As for other functions of covering materials, in response to the rapid progress of research on photomorphogenesis in the 1950s, studies on the utilization of light quality including the control of photomorphogenesis were actively carried out in Japan over a period of more than 20 years up to 1983, under the leadership of the Ministry of Agriculture, Forestry and Fishery. However, no practical result has been obtained as yet (Agriculture and Horticulture, Vol. 69, No. 9, p. 986, 1994).
At present, seedlings produced in seedling production facilities generally tend to be overgrown, and dwarf and robust seedlings are desired. Dwarf seedlings are resistant to natural environmental conditions (survive better in adverse climate) and are also effective in minimizing the lodging of plants and thereby preventing the loss of crop yield. In the cultivation of flowers, flowering plants having a long stalk are highly prized as cut flowers, whereas dwarf plants having large flowers tend to be desired for potted plants. In the cultivation of fruit trees, the dwarfed plants contribute to the improvement of workability. Moreover, in the case of graft seedlings, a uniform internode length is required for cutting by a robot. Thus, the elongation growth of plants affects their commercial value. Chemical regulation with a dwarfing agent or by mechanical retardation (i.e., training and pruning) is currently being exercised, but there is a need for a safer and more convenient method.
On the basis of experiments using artificial light sources, it has conventionally been known that the elongation growth of plants is promoted in a light environment containing a high proportion of far red light. The reason for this is presumed to be that an increase of far red light causes a reduction in r/fr ratio (in which r is the photon flux at 660 nm and fr is the photon flux at 730 nm), the reduced ratio changes the photostationary state of phytochrome, and this result acts as a signal to promote the elongation of plants. On the other hand, it is also known that the elongation of plants is retarded in a light environment containing a high proportion of red light. However, there has been no report dealing with the demonstration of these facts by use of a covering material. For example, films for controlling ultraviolet light and visible light are reported in Japanese Patent Laid-open Nos. 117738/'77, 132648/'89, 283212/'90 and 170322/'86, but there is no statement concerning the influence of far red light, relation with the r/fr ratio, elongation-promoting effect and the like. With regard to films for controlling far red light, their effect on specific types of plants has not been reported at all.
The present inventors have clarified that, in the evaluation, design and selection of an artificial light source for use in plant cultivation, not only the quantity of photosynthetically effective light (i.e., PPF luminous efficiency) but also the ratio of light in the red region to light in the far red region is important from the viewpoint of photomorphogenesis, and have concluded that a light source having high PPF luminous efficiency and a controllable red light/far red light ratio is desirable for plants. Moreover, they have shown that, when the red light/far red light ratio is used as an indicator for morphological control in an artificially illuminated environment for plant cultivation, it is most suitable to use a wavelength band of 600 to 700 nm for red light (R) and a wavelength band of 700 to 800 nm for far red light (Fr). Furthermore, they have made four-wavelength-region emission type fluorescent lamps having high PPF luminous efficiency and three levels of R/Fr ratio by way of trial, and have demonstrated its controlling effect on elongation growth (Murakami et al., Biological Environment Control, Vol. 30, No. 4, pp. 135-141, 1992). However, the use of these artificial light sources requires considerable equipment cost and operating cost (e.g., electric charges). Accordingly, a more inexpensive technique is needed in the existing circumstances.